1. Field of the Invention
The present invention relates generally to downhole tools for use in subterranean wells, and more specifically to a tool for use in cementing operations within a subterranean well.
2. Description of the Related Art
When a subterranean well, such as a well used in hydrocarbon development, is drilled it sometimes has a number of tubulars or casings of various diameters. The upper portion of the well have the largest diameter casing, which can be cased and cemented in one step. For deeper wells, such as, for example, wells that are 5000 feet deep or more, in the lower part of the well smaller diameter inner casing is run to the bottom of the drilled hole and cemented in two stages. The bottom most part of the casing is cemented using traditional means, such as by pumping cement through the casing shoe. An upper portion can be cemented using a second-stage tool, which is sometimes referred to as a multi-stage tool. The second-stage tool can be associated with the inner casing and includes an annular packer that seals the annular space between the inner casing and the bore of the well or an outer casing. The annular packer isolates a lower part of the annular space from the upper part of the annular space which is above the annular packer. The second-stage tool has ports that extend through the inner casing to allow circulating fluids to flow through the ports and into the upper part of the annular space.
The second-stage cementing process allows the cement to enter the annular space at a distance above the bottom of the well, preventing the entire hydraulic pressure of the weight of the column of cement being exerted on the bottom of the well. It also enables cementing the upper part of the hole in case a low pressure zone, known as loss circulation zone, is encountered in the middle portion of the well. The first step of the two stage cementing can be performed by pumping enough cement into the lower part of the annular space and up to the depth of the second-stage tool, or to the depth of loss circulation zone, as applicable. Then the annular packer is set to isolate the lower part of the annulus, and the ports are opened to circulate cement into the annular space above the annular packer. Mechanical and hydraulic means can be used to set, to open, and to close the second-stage cementing tools depending on the model of second-stage tool used.
Failure of the second-stage and off-bottom casing cementing process is common. For example, the second-stage tool can fail to open, leaving the annular space above the second-stage tool without cement. Such failure results in significant cost increases and time delays and affects the condition of the well. If alternate means for cementing the annular space are not possible, the upper part of the inner casing is left non-cemented, which will shorten the well life significantly. Traditional methods to mitigate the failure are complicated procedures and can result in substandard well conditions or can result in changing the well design to less than desired casing sizes and reducing well integrity.
As an example, whenever second-stage tool fails to open, a cement bond log can be run to determine the level of cement in the annular space. The inner casing can be perforated above the second-stage tool and a cement retainer packer can be run above the perforations. Cement can be circulated through the new perforations into the upper annular space. The perforations can then be scabbed off with smaller casing, which procedure is mandatory in some countries, or with casing patches where allowed. Forming the perforations risks damaging the next casing or the outer casing and introduces safety risks with the handling of explosive material required for the perforation gun. In addition, having to scab off the perforations will result in a reduced inner diameter of the inner casing, which can reduce the well production and injection rates and can limit options for future well interventions.
Alternately, for shallow wells, cement can be dumped into the annual space from the surface. This generally results in substandard quality cement.